1. Field of the Invention
This invention relates to an oily inkjet ink. This invention particularly relates to an oily inkjet ink, which has a good aptitude for inkjet and a good aptitude for toners, and which is free from the risk that an uncomfortable alcoholic odor will be given out from printed paper after image formation (after printing).
2. Description of the Related Art
Inkjet recording systems are the systems in which an inkjet ink having a high degree of fluidity is jetted out in the form of ink droplets from fine head nozzles, and in which an image is recorded by the ink droplets on printing paper located so as to stand facing the nozzles. Recently, the inkjet recording systems have rapidly become popular for the possibility of high speed printing with low noise. As the inks for use in the inkjet recording systems, there have been proposed various oily inkjet inks, which contain a water-insoluble solvent and a pigment finely dispersed in the water-insoluble solvent.
For example, in U.S. Pat. No. 7,799,123, the inventors proposed an ink, which contains a non-polar solvent, such as an ester solvent, a higher alcohol solvent, or a hydrocarbon solvent, and a pigment dispersed in the non-polar solvent. The proposed ink has the advantages in that the ink has excellent on-printer stability and a good aptitude for inkjet, and in that the ink yields a printed surface, which does not adhere to a print surface having been printed with a plain paper copier or a laser printer when being superimposed upon the print surface. The proposed ink thus has a good aptitude for toners.
With regard to the inkjet printing, the problems of odors are often encountered. Examples of the odors include an odor due to a residual monomer in a UV ink (as described in U.S. Patent Application Publication No. 20030063172) and an odor due to a solvent contained in a non-aqueous ink in which a printing medium is vinyl chloride (as described in Japanese Unexamined Patent Publication No. 2008-274034). However, with respect to oily inks used ordinarily, before now, problems of odors have not been reported.
However, recently, as for the inkjet printing using an ordinary oily ink which does not contain a specific substance that will be the cause of the odor as described above, the problems have occurred in that an uncomfortable odor is given out from the printed paper after image formation. The inventors have eagerly conducted research and have found that, of calcium compounds, such as calcium carbonate, calcium hydroxide, and calcium oxide, which are contained in paper, calcium hydroxide, which is susceptible to extraction by water and which has a high pH value, causes the problems of the odor to occur. Specifically, the inventors have found that calcium hydroxide contained in the printing paper has the effect of a catalyst upon ester solvents widely used in ordinary inks, e.g. isononyl isononanoate and isooctyl palmitate, hydrolyzes the ester solvents to generate alcohols having the uncomfortable odor, and thus causes the problems of the odor to occur. The present invention is based upon the findings described above.